disabling pan-tilt limits in PTU driver

a new private parameter `check_limits` (boolean) has been introduced which is set to *True* by default (keeping the default behaviour). With this limit in place, one can only pan up to +/- 2.8 rad. Setting it to false allows to do a full PI pan. This pull request also changes the default in the launch file to *not* check the limits as we want to turn our head fully reversed.